1. Field of the Invention
The present invention relates generally to preparing printed circuit boards for the mounting of semiconductor devices thereon. More particularly, the present invention is directed to the preparation and fabrication of printed circuit boards to reduce warpage caused by the application of epoxy encapsulant material placed upon the surface of the printed circuit board.
2. State of the Art
The fabrication of integrated circuits on areas of a wafer to form a discrete semiconductor die thereon is a long and complex process. One of the last steps in the process is that of encapsulating the semiconductor die as a semiconductor device and attaching the die to a printed circuit board (PCB) or other type die carrier.
Conformal coatings and encapsulants are typically applied as one of the last major processes of the fabrication of either the printed circuit board or other type of die carriers. In either case, the combination of the semiconductor device attached to a printed circuit board or other type die carrier has increased the value of the assembly over the value of the separate components. Therefore, the mounting of the semiconductor device and the encapsulation thereof on the printed circuit board or other type die carrier must have a high reliability and high yield, respectively.
Encapsulation of the semiconductor device protects the semiconductor device during any subsequent processing and prevents mechanical damage while providing protection from the operating environment for the semiconductor device.
Conformal coatings are used to encapsulate and protect various types of electronic packages, primarily from their operating environments. Specialized coatings have been developed to provide an enhanced protection from direct attack of hostile gases and liquids on critical surfaces of the packages. Polymeric films act only as semi-hermetic barriers because of reduced solubility or permeability of a hostile reactant in the polymeric material, or both.
Polyamides, polyamide-imids, and silicones have been developed for applications that can tolerate high cure temperatures and that need protection at elevated temperatures. These types of materials are most frequently used either directly on the semiconductor die at the die level, as passivating layers thereon or at the die carrier level. Polyurethanes, fluoropolymers, silicones, accrolades, and epoxies are most commonly used for components and printed circuit boards. These materials are typically applied from solution by emersion or spray coating or they may be applied via stencil coating or direct spreading. After curing, most coatings used on the semiconductor devices and a printed circuit board or other type carrier are difficult to remove because they become cross-linked during the curing process.
Materials typically used to encapsulate semiconductor dice mounted on various types of lead frames and to seal metal cans housing semiconductor dice and their carrier, as well as many other components, including potting and molding compounds as well as glob top encapsulants, must provide protection from handling damage for the semiconductor dice and their carrier in the post processing environment and any subsequent operating environment. Semiconductor dice are most frequently electrically connected to the lead frame by bonding wires between the bond pads on the semiconductor die and the leads of the lead frame (wire bonding). Flip-chips use small solder balls as interconnects to a substrate and tape automated bonding using thermal compression bonding to form interconnections between the circuits located on the tape and the bond pads of the semiconductor die. Interconnections between substrates and semiconductor dice, as well as other components, are fragile and subject to stress failures. The encapsulant must not generate catastrophic stresses due to the chemical curing process of the encapsulant material or stresses due to differing rates of thermal expansion of the semiconductor die, substrate, and encapsulant during the thermal cycling thereof.
Initially, rigid epoxies were primarily used for encapsulation. Epoxies have the advantages of relatively little shrinkage, high resistance to processed chemicals, and good mechanical properties. Since semiconductor device package sizes are growing, highly filled epoxies with reduced thermal expansion have been developed to reduce stresses in these packages.
Unfortunately, even the best of epoxies still has some level of shrinkage that results in warpage of the underlying substrate, such as a printed circuit board (PCB). The warpage of a printed circuit board can stress the board enough to either cause it to fail or to cause any of the attached semiconductor devices to fail. Failure of semiconductor devices typically occurs because the solder links between the semiconductor device and the circuits on the printed circuit board failed due to the stress caused by the warpage of the board. Conformal coatings may also incur stress on a surface mounted chip (SMC) during thermal cycling of the chip and printed circuit board causing the solder joints to crack or the components to fracture. Differences between the coefficients of thermal expansion of the encapsulant, the coating, the printed circuit board, and a semiconductor device mounted thereon cause greater stress during thermal cycling. A coating that has a coefficient of thermal expansion (CTE) nearly matching that of the substrate and the semiconductor devices mounted thereon will produce less stress therebetween and attendant cracking when subjected to thermal cycling. Larger surface-mounted chips are more vulnerable to damage from stresses during curing of the encapsulant material and thermal cycling of the chip and substrate due to the differences in the coefficient of thermal expansion of the chip and substrate causing stresses therebetween.
The thicker the coating or encapsulant thickness of a semiconductor device, the greater the likelihood of stress on the semiconductor device and its connections or interconnects to the substrate from shrinkage of the coating or encapsulant. Some surface-mounted chips may not be able to withstand mechanical stresses induced during curing of thick coatings, which may also result in the warpage of the printed circuit board upon which the chip is mounted. If the solder interconnections between a semiconductor device and the circuits of a printed circuit board are closely spaced, conventional coating materials and encapsulant materials may move the semiconductor device, thereby cracking the solder joints as such material cures. In addition, thicker material coatings or thicker encapsulant material may act as barriers to heat transfer from densely packed surface mount chips during the operation thereof.
The present invention is directed to a method and apparatus for preventing board warpage during the application and curing or drying of liquid epoxies, or the like, on printed circuit boards. A clamping fixture assembly, which includes at least one clamping fixture support and at least one clamping fixture overlay, is used to restrain the printed circuit board on a flat surface during the curing of the epoxy. If desired, a plurality of printed circuit boards may be processed using an appropriate clamping fixture assembly. Furthermore, the clamping fixture may be constructed so a slight bow or curvature thereof can counter either a convex or concave bow or curvature of the printed circuit board.
In the method, at least one printed circuit board is mounted to a clamping fixture support where a clamping fixture overlay is placed on top of the first printed circuit board. Next, an aperture in the clamping fixture overlay allows for the application of an encapsulation material, such as an encapsulant epoxy, to be spread within an area bordered by an epoxy dam. Next, the epoxy is cured or dried on the printed circuit board. Such curing or drying can be performed within an oven for a predetermined period of time at a predetermined temperature sufficient to optimize curing or drying of the epoxy without excessive board warpage, such warpage being limited by the printed circuit board being retained in the clamping fixture assembly.